- SPECIAL REPORTS
Given such high rates of production, you might be tempted to think “haste makes waste,” but think again. Even when faced with breakable, bendable or scratchable parts, engineers have developed ingenious ways of meeting the desired production rate.
Most parts put together on automated assembly systems are reasonably durable. Put hundreds of steel screws or plastic clips into a vibratory feeder bowl, and they’ll be fine. Other parts require a more delicate touch, literally and figuratively. Glass and ceramic parts, wire forms, thin metal stampings and other fragile or delicate parts can present a challenge to high-speed automated assembly.
“The fragility of a part is not necessarily determined by the material,” explains Paul Gray, lead development engineer at Edgewater Automation. “A fragile part would be one whose shape or surface finish is easily damaged in the process of feeding, assembling or testing. So the features that would alert us to a potential problem would be a part that is soft or thin, or a part that has features that are precise and easily damaged.
“Parts that are brittle or that don’t hold their shape can be especially difficult. We want the parts to hold up to the rigors of automation. We work hard to match the automation with the part characteristics to keep from damaging fragile parts.”
In the world of automation, assessing the fragility of a part is not always easy. Parts that seemingly should be fragile actually hold up well in automation.
“Some parts are a lot more robust than they look,” says Bob Rice, applications team leader at ATC Automation. “Bundles of glass tubes for fluorescent bulbs are fed with a sling. The bundle is coarsely released to a conveyor without breakage. You would anticipate serious breakage, but they don’t.”
Jeff Stover, sales manager at Xigent Automation Systems Inc., agrees. His company has designed systems for handling glass vials, plate glass and semiflexible mirrors.
“Glass is actually a pretty rugged material,” he says. “You just can’t drop it or slam it into something. We recently worked on a material handling application in which we had to retrieve glass vials from a washing operation, 10 at a time, place them into stainless steel pucks, and pass them in front of an inspection station. We were able to run that at a 2-second pick-and-place cycle.”
Conversely, parts that seem robust can be fragile if not stored, handled and fed correctly. For example, a Midwestern manufacturer of felt-tip pens gets plastic caps from a local injection molder. The molder ships the caps in boxes of several thousand. If a box sits too long, some of the caps at the bottom eventually get deformed under the weight of the ones on top. The bent caps then cause jams in vibratory feeders.
“Many times, a rib or thicker walls can be added to a part without a lot of extra cost or changing its function,” says Gray. “We have also added assembly aids to the part to help a fragile part go together without adding too much additional cost. The return on investment will come from greater throughput or higher yields.”
Fragility is a relative term, adds Gray. For example, a part might be very robust, but have a precision finish. In that case, the concern is more about caring for the surface than whether the part will break during the assembly process.
“When most people think of fragile parts, they think of their grandmother’s tea set,” says Gray. “However, in automation, it might be easier to handle a glass part than a steel part with a critical surface finish.”
Feeding Fragile Parts
Assemblers have many options for feeding fragile or delicate parts.
For starters, the vibratory feeder is still an option for many parts, says Greg Pflum, president of Performance Feeders Inc. To protect the surface finish on the parts, the bowl’s metal running surfaces can be coated with a polymer or lined with urethane, rubber or Brushlon. As a bonus, such measures also attenuate the noise generated by the feeder. To limit damage from parts rubbing against each other, assemblers may want to limit the number of parts in the bowl at any one time.
Such practices came into play recently with a bowl Performance Feeders made to feed uncirculated coins for resale. “We applied 3/16 Brushlon on all the running surfaces, and used Delrin or UHMW polyethylene on all other part-contact surfaces,” recalls Pflum. “We also used noncontact level sensors to maintain a low part level in all portions of the system.”
If the feed rate must be reduced to prevent damaging the parts, the bowl can be designed to supply multiple feed lanes. That way, the final output is the same, even if the bowl itself is running slower. If that’s not an option, the part could also be fed from more than one bowl.
“We can adjust the tooling so that misoriented parts are repositioned to be good parts without having to be recirculated,” adds Pflum. “Feeding the parts directly from the prefeeder to the return pans can lower the drop height from 8 to 12 inches to 0.5 inch.
“You can also consider different types of feed systems. A vibratory feeder works for most applications, but there are times when a centrifugal feeder, a step feeder or an orienting elevator is a better option. It’s important to look at all available feeding options when you have a fragile part.”
One alternative is tray feeding, though it can be expensive, says Rice. Custom-made, vacuum-formed plastic trays can be made in sizes ranging from 3 inches square to 4 feet square. Trays can be vacuum-formed from PVC, PET, ABS or styrene in thicknesses ranging from 0.015 to 0.375 inch. Cavities for holding parts can be made in virtually any size or shape. Trays can also be supplied with locating holes, engravings and matching covers.
Fragile parts can also be fed from a magazine or tape-and-reel. ATC recently worked on a system in which small glass bulbs are fed from a coil and handled with rubber-coated grippers.
Yet another option is to feed the parts directly from the downstream process, Gray points out. That saves the cost of dunnage, and it saves time. Once you have control over the part, why give it up by dumping it into a bin?
Gripping Fragile Parts
Vacuum cups are the simplest and least expensive method of gripping parts. There are cups for parts weighing a few ounces and for parts weighing several hundred pounds. There are cups for gripping parts with odd surfaces, such as golf balls, and cups for gripping parts with odd shapes, such as tubes. Check valves ensure that the cups won’t drop the part in the event of a power loss.
More importantly, vacuum cups are soft and will not mar fragile surfaces. ATC has used vacuum cups to handle metal parts as thin as 0.003 inch thick.
Another soft-touch option is a bladder clamp, which is essentially a rubber donut that inflates and deflates on command. The clamp is placed over and around the object to be picked up. When the bladder is inflated, it applies a gentle, even pressure around the part. Bladder clamps have been used to handle ice cream cones, graphite electrodes and even precision mirrors for telescopes.
If traditional grippers are used to handle fragile parts, rubber or silicone sleeves can be placed over the fingers.
Even in the best-designed assembly system, glass parts will break and wire leads will bend. Engineers are
well-advised to plan for the inevitable broken part.
“The best advice for handling fragile parts is to perform a thorough failure modes and effects analysis,” says Gray. “Understand how the part might be damaged, what the effect would be, how failures will be detected, how damage can be prevented, and how damaged parts will be removed from the process.
“If you know your parts are susceptible to damage, plan on what will happen when the part does break. Make sure it doesn’t jam the machine. Self-cleaning features in the tooling can be a big help for assembling fragile parts.”